Re: generics:< ? >vs.< T >

From:
Robert Klemme <shortcutter@googlemail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 9 Mar 2011 06:11:10 -0800 (PST)
Message-ID:
<506400f3-c294-4df7-8a46-de5450fe5a7b@glegroupsg2000goo.googlegroups.com>
On Wednesday, March 9, 2011 1:32:17 PM UTC+1, Lew wrote:

Generics are tricky because we tend to think of them as instructions. Th=

ey

are not.
 
'<?>' does not mean "any type"; it means "an arbitrary but unknown subtyp=

e of

Object". '<T>' doesn't mean "any type"; it means "a particular inferrabl=

e

(therefore known) type". The biggest difference is that the wildcard doe=

s not

assert which particular type is in play, but the type parameter does.
 
That's why they're incompatible. No way the compiler can assert that som=

e

unknown wildcard type (the capture of the wildcard's bound) is reliably=

 

compatible with the assertrf type 'T'. We just don't know which type the=

 

wildcard represents and cannot match it to 'T'.


You also see that from error messages where wildcard captures are numbered =
so each position of <? extends Foo> gets its own calculated type and of cou=
rse it is *not* true that for each A extends Foo and B extends Foo: A can b=
e assigned from B or B can be assigned from A.

I had to puzzle out a related issue a few days back. I had a method with t=
his signature

  public static <V2, V1 extends V2> void copy1(final Map<?, V1> source,
      final Map<? super String, V2> target) {
    // work
  }

Then, for convenience reasons I wanted parameter "target" to be returned in=
 order to be able to do Map<...> m = copy(..., new HashMap<...>, ...) in =
one line.

First solution was

  public static <V2, V1 extends V2> Map<? super String, V2> copy2(final Map=
<?, V1> source,
      final Map<? super String, V2> target) {
    // work
    return target;
  }

This compiles but when used in a line like this

final Map<String, MapsTest> m2 = copy2(source, new HashMap<String, MapsTe=
st>());

compiler errors out with

Type mismatch: cannot convert from Map<capture#2-of ? super String,MapsTest=

to Map<String,MapsTest>


The solution was to mention the type of "target" and return value explicitl=
y:

  public static <V2, V1 extends V2, M extends Map<? super String, V2>> M co=
py3(
      final Map<?, V1> source, final M target) {
    // work
    return target;
  }

Now the compiler knows that both are of the same type and assignment doesn'=
t error out.

Btw, has anybody an explanation why I can do copy1() from above but not

  public static <V1, V2 super V1> void copy0(final Map<?, V1> source,
      final Map<? super String, V2> target) {
    // work
  }

? This errors out with a syntax error at "super" between "V2" and "V1".

Cheers

robert

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